Better Image Viewer von Geoff Lankow

Better Image Viewer adds the following actions:

Scroll up/keyboard plus: zoom in
Scroll down/keyboard minus: zoom out
Keyboard zero: zoom 1:1
Click: toggle between zoom-to-fit and 1:1
Drag: pan
Shift+drag: drag image (e.g. to another window)

You can also use a popup toolbar on the left-hand side of the window to control zoom.

Better Image Viewer is no longer maintained.